1. Field of Application
The present invention relates to a direction detection apparatus which estimates the direction of arrival of electromagnetic waves, based on signals received by an array antenna.
In particular, the invention relates to a direction detection apparatus implemented as a radar apparatus installed on a vehicle, which transmits radar waves and detects the directions of target bodies based on respective directions of arrival of reflected waves from the target bodies.
2. Description of Related Art
Types of radar apparatus which perform direction detection are known, having an array antenna for receiving radar waves, with the array antenna formed of a plurality of antenna elements. Radar waves transmitted from the apparatus are reflected from a target body, and respective received signals resulting from the radar waves are obtained from the antenna elements. The direction of arrival of the radar waves, and hence the direction from the radar apparatus to the target body, is estimated based upon these received signals. Various methods are known for accomplishing this, including beam scan methods such as digital beam forming (DBF) and null scan methods such as MUSIC (MUltiple SIgnal Classification). With a beam scan method, the direction of each target body is estimated by using a corresponding main lobe of the received signal pattern obtained by the array antenna, so that the direction detection resolution is determined by the beam width of the antenna. With a null scan method, since null points of an array antenna having a narrow half-angle are utilized, a high degree of direction detection resolution can be achieved. This is described for example in “Adaptive Signal Processing by Array Antenna” (pages 194-199) published in 1998 by Kagaku Gijutsu Shuppan in Japan, authored by Nobuo Kikuma, referred to as reference document 1 in the following.
In the case of a radar apparatus that is be installed on a vehicle (referred to in the following as the “local vehicle” for brevity of description) for detecting objects such as other vehicles in the environment, the space available for installing the system is extremely limited. The overall size of radar system is mainly determined by the size of the antenna apparatus, so that it is desirable to make the antenna as compact as possible.
Diagram (a) of FIG. 5, illustrates the case of a radar apparatus which utilizes a beam scan method for detecting directions of target bodies based on detecting main lobes of a received signal strength/direction spectrum (DBF spectrum), i.e., a pattern of levels of received signal strength obtained from respective elements of an array antenna of the radar apparatus. With such a method, the problem arises that since the beam width of the array antenna is large, the direction detection resolution is low. Hence it may be impossible for the apparatus to distinguish the respective directions of a plurality of target bodies when the target objects are close together. Moreover if the antenna size must be made small (i.e., the antenna aperture is small), then the beam width of the antenna becomes increased, so that the resolution of direction detection based upon main lobes of the received signal pattern becomes correspondingly lowered.
With a null scan type of algorithm such as MUSIC on the other hand, even if the antenna is made small in size, a high resolution of direction detection can be achieved. However when the difference between the velocities of the target objects is very low or zero, e.g., as occurs when the local vehicle and target objects are halted, it becomes difficult for the apparatus to accurately detect the directions of the target bodies (as illustrated in diagram (b) of FIG. 5).
That is to say, if the target bodies and the local vehicle are stationary, so that there is no difference between the velocities of the target objects, the respective reflected waves arriving from the target bodies will have a strong correlation. Specifically, the difference between the respective phases of the reflected waves from any pair of target objects will not vary with time. With a null scan type of algorithm such as MUSIC, the directions of arrival of incident reflected waves are obtained based on a correlation matrix referred to herein as the observed matrix, which is established based on the received signals from the respective elements of the array antenna. Considering the received signal obtained for each antenna element, when the signal contains components corresponding to a plurality of incident reflected waves (arriving from respectively different directions), and these signal components have a strong correlation, the system will judge that the reflected radar waves are arriving along a single direction, i.e., from a single target body. Hence the respective directions of target bodies cannot be accurately detected by using an algorithm such as MUSIC. Here, “correlation” signifies that the phase difference between a pair of such signal components (constituting a received signal from an antenna element) does not vary with time.
As described in pages 247-263 of reference document 1 above, a method of suppressing the correlation between incident radar waves arriving from different directions is known, whereby a correlation matrix is generated by using spatial averaging. With this method, designating the number of antenna elements of an array antenna as M, and the number of antenna elements of a sub-array as K (<M), respective antenna signals are obtained from sub-arrays that are respectively shifted by one element position, i.e., from a total of N sub-arrays (where N=M−K+1). The required correlation matrix (utilized in estimating the desired directions) is obtained by spatial averaging of correlation matrixes that are derived based on the sets of antenna signals obtained from the respective sub-arrays.
That is to say, the phase relationship between the incident waves arriving from different directions is determined by the reception point of the waves. By deriving respective correlation matrixes that are obtained by successively shifting the reception point, and using averaging of values in these matrixes to derive the values for a final correlation matrix, the effects of correlation between the incident waves upon the final correlation matrix can be suppressed.
However the number of different directions which can be detected is determined by the order of the correlation matrix, i.e., by the number of antenna elements constituting the array antenna. If spatial averaging of respective correlation matrixes obtained from a plurality of successively shifted antenna sub-arrays is utilized as described above, then the problem arises that the number of target bodies whose directions can be separately detected will be reduced.
In particular, if the radar apparatus is to be made compact in size, so that the array antenna must be small, then this becomes a serious problem.